1. Separation of liquid mixtures
Separation of liquid dispersions or mixtures may be achieved by a number of techniques, including simple gravity separation (decantation), centrifugal separation, impaction methods such as coalescing or flotation, and electrotreating. Of these, by far the cheapest and simplest under most circumstances is decantation. For decantation to be effective, however, the concentration of the minor phase should be relatively high, and the dispersed phase should be present in the form of relatively large droplets. Also, decanters can typically achieve only a gross separation between phases. Even when an optimum phase separation is achieved, the dispersed phase may be slightly soluble in the continuous phase, so that a total separation of the two components is not possible. Therefore, it is frequently necessary to subject the residue from the decanter to some form of secondary separation process. Where large volumes of liquid are to be treated, the space required for the decanter vessels may be a problem. Thus, to take advantage of the cheap, simple aspects of decantation as a separation process requires:
1. A relatively concentrated mixture, PA1 2. A relatively small feed volume, if possible, and PA1 3. A second polishing step for optimum separation.
2. Treatment of streams containing dissolved organics
Contamination of industrial effluent waters with dissolved organic solvents, such as methanol, ethanol, methyl ethyl ketone, phenol, benzene, toluene, and trichloroethane, is an important environmental problem. These solvents make water unfit for reuse or direct discharge to municipal sewers, and are difficult to remove, even at low concentrations. Commonly used methods for removing volatile organic compounds include air stripping, biological treatment, carbon adsorption and incineration. Air stripping, in which water is circulated against a current of air in a contacting tower, is the least expensive process. However, air stripping merely exchanges water pollution for air pollution. Air stripping is therefore seldom used if the solvent concentration exceeds 0.1%, and even then only for small streams where the total organic emission is less than 10-100 lb/day. Carbon adsorption, a principal effluent treatment technology, can only be used efficiently for very dilute streams, typically 1,000 ppm or less, and more usually 100 ppm or less. At these very low concentrations, carbon adsorption is a preferred technique, because the size of the plant scales in proportion to the amount of solvent removed. Thus, when the solvent concentration is very low, the amount of wastewater that can be treated by a small carbon adsorption system is high. However, once the feed solution concentration exceeds 1,000 ppm, carbon adsorption systems become very large per gallon of wastewater treated. Also, carbon adsorption systems cannot handle some chlorinated and fluorinated solvents, and generate secondary waste, in the form of spent contaminated carbon that may be sent to landfills. Biological treatment systems work well only for organics that can be fully metabolized by the biomass, and where the process is not compromised by high or fluctuating solvent concentrations. At the high concentration end of the scale, incineration is reliable and effective for very concentrated streams, where the heat value of the solvent reduces the amount of supplemental fuel required. Typically, incineration is impossibly expensive at concentrations below 5%.
Thus, there is a dissolved organic concentration range for which no conventional wastewater treatment method is really suited. Currently this range is avoided by pooling or diluting the waste to the point where it can be treated by carbon adsorption or biological processes, or concentrating it so that incineration can be used. There remains a real and long-felt need for a low-cost method for directly treating contaminated streams, with an organic content of about 0.1% up to about 5 or 10%, to produce an effluent suitable for direct discharge or biological treatment, and a low-volume concentrated stream containing the bulk of the dissolved organics, from which the organic component can be recovered. Likewise, there is a need for efficient methods of separating organic/organic process streams or wastes, particularly where the two organics form an azeotrope or a closely-boiling mixture.
Pervaporation is a relatively new separation technology that is beginning to achieve commercial success. The pervaporation process itself has been known since the 1950s, and is described, for example, in U.S. Pat. Nos. 2,913,507 and 2,953,502 to Binning et al. Organic-selective pervaporation is described in a general way in U.S. Pat. No. 4,218,312 to Perry. Despite the theoretical knowledge embodied in these patents, many years elapsed before commercially viable pervaporation systems could be contemplated, because the technology to make high performance membranes and modules had not been developed. In recent years, a West German company, GFT GmbH, has been selling pervaporation units with water-selective membranes to remove small amounts of dissolved water from solvents. Membrane Technology & Research, Inc., now offers systems with organic-selective membranes for a variety of applications. Work on organic/organic separations by pervaporation is ongoing in several institutions.